Restonic Complaint - Sagging Restonic Bed

We bought a King size Restonic about 3 years ago. The mattress keeps sagging on both sides and humped in the middle. We have tried to rotate it but still sags. We finally had to turn it over and put cushioning on the mattress to sleep on it. We paid over $1000.00 for it and the store where we purchased has gone out of business.